1. Field of the Invention
The present invention relates to a MEMS gyroscope, and more particularly, to a MEMS gyroscope having a mass vibrating vertically on a substrate.
2. Description of the Related Art
MEMS (micro electro mechanical system) is a technology realizing mechanical and electric elements using a semi-conductor process. One example of an element using MEMS technology is a gyroscope for measuring angular velocity. The gyroscope measures the angular velocity by measuring Coriolis force generated when rotation angular velocity is added to an object moving with a predetermined velocity. The Coriolis force is proportional to a cross product of the rotation angular velocity due to external force and moving velocity.
For the gyroscope to generate and sense the Coriolis force, the gyroscope has to have a mass vibrating inside of the gyroscope. Hereinbelow, a direction that the mass in the gyroscope is driven will be referred to as a ‘driving direction,’ a direction that the rotation velocity is input into the gyroscope will be referred to as an ‘input direction,’ and a direction that the Coriolis force generated in the mass is sensed will be referred to as a ‘sensing direction.’
The driving direction, input direction and sensing direction are all set up in a perpendicular direction to each other. Generally, in the gyroscope using the MEMS technology, three axes of coordinates are set up composed of two directions crossing at a right angle and parallel to a surface of a substrate (hereinbelow, it will be referred to as a ‘horizontal direction’) and a direction vertical to the surface of the substrate (hereinbelow, it will be referred to as a ‘vertical direction’).
Generally, the gyroscope is divided into an X type (or Y type) gyroscope and a Z type gyroscope. The input direction of the X type gyroscope is the horizontal direction. To measure the angular velocity added in the horizontal direction by using the X type gyroscope, either the driving direction or the sensing direction should be set up as vertical direction. Therefore, the X type gyroscope should have a driving electrode to drive the mass vertically or a sensing electrode to sense vertical displacement of the mass.
In the gyroscope using the MEMS technology, the driving electrode and the sensing electrode have the same physical structure. FIG. 1 is a view showing a driving electrode of vertical direction or a sensing electrode of vertical direction.
A stationary electrode 11 is mounted on the substrate 20 and a moving electrode 13 is disposed above the stationary electrode 11. The moving electrode 13 is lifted above the substrate 20 in order to be able to approach and separate to and from the stationary electrode 11.
When the above described electrode is used as a driving electrode, the voltage added between the stationary electrode 11 and the moving electrode 13 is variable, and accordingly, the moving electrode 13 is vertically vibrated in relation to the stationary electrode 11 by variable electrostatic force. When the above electrode is used as a sensing electrode, a means to sense the electrostatic force change based on the distance between the stationary electrode 11 and the moving electrode 13 is provided. The moving position of the moving electrode 13 can be obtained according to the sensed result, and Coriolis force can be obtained based on the position.
Yet, the electrode having the above structure is very difficult to manufacture as the moving electrode 13 is suspended above the upper part of the stationary electrode 11. In other words, to manufacture the above electrode, a process for forming the stationary electrode 11 on the substrate 20 is firstly operated, and then a sacrificial layer is deposited on the stationary electrode 11. After that, the moving electrode 13 is formed on the sacrificial layer, and the sacrificial layer is removed. As it is known from the above, the process to manufacture the moving electrode 13 suspended above the stationary electrode 11 involves many steps.
Moreover, to correctly measure the displacement of vertical direction of the moving electrode 13, the distance between the moving electrode 13 and the stationary electrode 11 should be narrow. Therefore, there is a problem that adhesion can be created between the moving electrode 13 and the stationary electrode 11.
Accordingly, the MEMS gyroscope having a driving electrode and a sensing electrode shown in PIG. 1 requires many steps in manufacturing and has a high possibility of malfunction due to the adhesion.